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Search: id:A156759
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| A156759 |
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a(1)=2, a(n+1) is the smallest composite number with smallest prime factor >= prime factor of a(n). |
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+0
2
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| 2, 4, 6, 8, 9, 15, 21, 25, 35, 49, 77, 91, 119, 121, 143, 169, 221, 247, 289, 323, 361, 437, 529, 667, 713, 841, 899, 961, 1147, 1271, 1333, 1369, 1517, 1591, 1681, 1763, 1849, 2021, 2209, 2491, 2773, 2809, 3127, 3233, 3481, 3599, 3721, 4087, 4331, 4453, 4489
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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Apart from a(1), this is a sequence of increasing composites such that the derived sequence of their least prime factors is non-decreasing. [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Feb 20 2009]
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EXAMPLE
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a(1)=4=2*2(2=2), a(2)=6=3*2(3>2) where 2=2, a(3)=8=2*2*2(2=2=2) where 2=2, a(4)=9=3*3(3=3) where 3>2, a(5)=15=5*3(5>3) where 3=3, a(7)=21=7*3(7>3) where 3=3, a(8)=25=5*5 where 5>3, etc.
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MAPLE
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A020639 := proc(n) min(op(numtheory[factorset](n))) ; end: A156759 := proc(n) option remember ; local a; if n = 1 then 2; else for a from procname(n-1)+1 do if not isprime(a) then if A020639(a) >= A020639(procname(n-1)) then RETURN(a) ; fi; fi; od: fi; end: seq(A156759(n), n=1..100) ; [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Feb 20 2009]
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CROSSREFS
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Cf. A002808, A156604.
Sequence in context: A074901 A125990 A080223 this_sequence A114973 A004522 A004521
Adjacent sequences: A156756 A156757 A156758 this_sequence A156760 A156761 A156762
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KEYWORD
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nonn
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AUTHOR
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Juri-Stepan Gerasimov (2stepan(AT)rambler.ru), Feb 15 2009
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EXTENSIONS
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Corrected by R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Feb 20 2009
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